One way is to look at it is to consider it to be made up of two arithmetic sequences: Sequence 1: 163, ,949, ,1735, ,2521, ,3307, ,4053, ,... Sequence 2: ,524, ,1898, ,3272, ,4646, ,6020, ,7394,... Together: 163,524,949,1898,1735,3272,2521,4646,3307,6020,4053,7394,... Sequence 1 has first term 163, and common difference 786. Sequence 2 has first term 524, and common difference 1374. Edwin